In recent years, an exposure method using an extreme ultra violet ray (hereinafter abbreviated as “EUV” for Extreme Ultra Violet) is suggested. The wavelength of an EUV exposure light is small. As a result, the refractive index of a material is close to the value of a vacuum. In addition, there is only a small difference between the light absorption of different ingredients. Therefore, in an EUV wavelength region, a traditional transparent dioptric system cannot be made. Thus, a catoptric system is attained. Hence, the photomask becomes a reflective photomask.
According to a general reflective photomask developed so far, a configuration was applied such that a high reflection part and a low reflection part are provided on a Si wafer or a glass substrate. The high reflection part includes a multi-layered reflection film and a capping film. The multi-layered reflection film is provided by stacking approximately 40 sets of double-layered films including Mo and Si, for instance. The capping film protects the multi-layered reflection film. The low reflection part is formed as a pattern of an absorption film and a buffering film on the high reflection part. The buffering film is a film acting to mitigate a damage on the capping film and the multi-layered reflection film when dry etching on absorption film and a repair on a defect by an ion beam or a laser are made. The buffering film is patterned after the defect on the absorption film is repaired.
According to a reflective photomask as described above, the low reflection part, which absorbs an EUV light, is normally configured to have a low reflectance with reference to a deep ultraviolet ray (hereinafter abbreviated as “DUV” for Deep Ultra Violet) in order to attain a contrast at the time of a pattern defect inspection. The DUV is a defect testing light. In order to attain a low reflectance, an anti-reflection (hereinafter referred to as “AR” for Anti Reflection) effect is utilized. This AR effect uses the so-called thin film interference. Therefore, on the top-most layer of the low reflection part, a film having a transparency with reference to DUV light is formed as an AR film.
In this way, the low reflection part is configured as a laminated structure such that various films like an AR film, an absorption film, and a buffer film, etc., are stacked in this order from a side farther from the substrate. The AR film attains a low reflectance with reference to a DUV light. The absorption film absorbs an EUV light. The buffering film mitigates damages to the capping film and the multi-layered reflection film.
Further, the capping film may also act as a buffer film when the capping film has transparency with reference to an EUV light, while at the same time having a high durability with reference to the dry etching of the absorption film, and when the repair of a defect of the absorption film is performed using an electron beam imposing only a small amount of damage. Such a capping film is called a dual purpose film.
Conventionally, a layer having Ta and Cr as primary components is formed as an absorption film of a reflective photomask for an exposure method using the EUV light (See Patent Document 1).
Not limited to the case using an EUV exposure, a first optical characteristic, required of a photomask to achieve a transfer exposure by a projection exposure, is a mask contrast. Normally, according to a transparent type mask, the mask contrast is represented by the following Equation (1). Here, T indicates the intensity of the transparent light which passed through the transparent substrate. To indicates the intensity of the transparent light which passed through a patterned part including a light shielding film.OD=−log(To/T)  (1)Here, OD refers to an optical density (Optical density). OD indicates the degree of light blocking effect of the light shielding film.
The mask contrast may be evaluated in a similar manner in the case of the reflective photomask. Since this mask is a reflective photomask, the mask contrast is evaluated by the following Equation (2), similar to the case of the transparent type mask. Here, Rm refers to the intensity of the reflected light from a high-reflection part. Ra refers to the intensity of the reflected light from a low-reflection part including the absorption film.OD=−log(Ra/Rm)  (2)
In general, in order to perform a good EUV transfer, it is preferable that the OD be greater than or equal to 1.5.
An EUV exposure is a reflective exposure. As a result, the incident light enters the reflective photomask not from a perpendicular direction, but from a slightly slanted direction (usually approximately 6 degrees), and becomes a reflected light. At this time, the EUV light is incident from a slanted angle. As a result, due to the patterned low-reflection part, a shadow of a pattern is created. Therefore, depending on the direction of the incidence and a direction in which the pattern is aligned, a deviation from an intrinsic position occurs at the transferred resist pattern on the wafer created with the reflected light. In this way, the accuracy of the pattern position deteriorates. This effect is referred to as a shadowing effect (Shadowing Effect). There is a problem with the EUV exposure in that this shadowing effect should be restrained.
In order to restrain the shadowing effect, the length of the shadow may be reduced. Thus, to reduce the length of the shadow, the height of the pattern may be made to be as small as possible. In other words, it is preferable to form the low reflection part as thin as possible.
In order to retain a mask contrast of OD>1.5 even though the low reflection part is made thinner, it is important to use a material having a large absorption characteristic with reference to the EUV light.
Further, in general, not limited to the reflective photomask, a photomask is repeatedly exposed to a cleansing liquid using acid and alkali and the like during the manufacturing process of the photomask and during the usage period in a transfer exposure. It is preferred that a thin film included in the mask have adequate durability against these cleansing liquids. Further, it is preferable that the thin film, which is to be patterned, have an adequate etching suitability (etch rate) so that a minute pattern may be formed on the thin film.
From above, according to a reflective photomask, in order to obtain film thickness of the patterned part is as thin as possible and also obtain a preferable mask contrast of OD>1.5, it is preferable that the low reflection part have a layer including a material having a high EUV light absorption characteristic, a high durability against cleansing liquid, and a high suitability for etching. However, a favorable film material satisfying these conditions has not been suggested.
Meanwhile, a technique for enhancing resolution using a phase shift mask has been suggested. A phase shift mask provides a phase difference of 180 degrees to a light that has passed through the phase shift mask. The transparent part of the mask pattern has a different material or a different shape compared to an adjacent transparent part. Therefore, in a region between these transparent parts, transmission diffraction light having a phase difference of 180 degrees cancel out each other. As a result, the intensity of the light becomes extremely small. Thus, the mask contrast is enhanced. Consequently, the focal depth of at the time of transfer expands. At the same time, the transfer accuracy enhances as well. Incidentally, in theory, it is ideal to have a phase difference of 180 degrees. However, the effect of enhancing resolution may be obtained as long as the phase difference is essentially 175 degrees to 185 degrees.
A half tone type is a type of a phase shifting mask. As a material included in a mask pattern, the half tone type uses a thin film which is semipermeable with reference to an exposure light (hereinafter referred to as a half tone film). Thus, the transmittance may be diminished to approximately several percent (usually, the transmittance is approximately 4% to 15% with reference to a light passing through a substrate). Further, a phase difference of approximately 175 degrees to 185 degrees may be provided with reference to the light passing through the substrate. As a result, the phase shift mask enhances the resolution of the pattern edge part. Further, the phase shift mask enhances the transfer accuracy.
Here, an appropriate range of a transmittance of a half tone type phase shift mask is described. According to a conventional half tone type mask for an excimer laser, it is preferable that the transmittance of the half tone film satisfy an optical condition of 4% to 15% in general with reference to an ultraviolet ray having an exposure wavelength. This is because, first of all, when the transmittance of the half tone film at the exposure wavelength is less than or equal to 4%, and when there is an overlap in the diffracted light of the light that has passed through the adjacent transparent part, the cancelling effect becomes small. On the other hand, when the transmittance is greater than or equal to 15%, the resolution limit of the resist is exceeded depending on the exposure conditions. Thus, an extra pattern is formed in a region of the half tone film through which the light passed.
The EUV exposure uses a catoptric system. Since an NA (Numerical Aperture) is small, and since the wavelength is short, the EUV exposure has a unique problem in that the EUV exposure is susceptible to the roughness on the surface of the mirror and the mask. Thus, it is not easy to resolve minute line widths with the targeted accuracy. Therefore, a half tone type reflective photomask is suggested (see, for example, Patent Document 2), which makes the absorption film act as a half tone film as well, in order to allow the principles of the half tone type mask to be applied to the EUV exposure as well. This EUV exposure uses a catoptric system. The principles of the half tone type mask are used in a traditional excimer laser exposure and the like.
The principles for enhancing the resolution with a phase shift effect are the same for a reflective photomask as well. The “transmittance” described above is simply replaced with “reflectance.” The appropriate values of the transmittance and the reflectance are considered to be approximately the same. In other words, it is preferred that the reflectance at the low reflection region with reference to the high reflection region be 4% to 15%.
Incidentally, using the half tone type photomask is in principle an effective way to enhance the resolution in lithography. However, normally, the most appropriate transmittance and the reflectance of a half tone type photomask relies on exposure conditions and the pattern that is transferred. Thus, it is difficult to determine a certain value.
Therefore, a configuration is preferred such that the reflectance may be selected (freely) in the range of approximately 4% to approximately 15%, even in the case of a half tone type reflective photomask when a light reflected from a patterned low reflection part has a phase difference of 175 to 185 degrees with reference to a light reflected from a high reflection part.